With iOS 6, you can choose to limit (though not entirely disable) ad tracking, by digging deep into settings. Go to Settings > General > About, then scroll down to the bottom and top on Advertising, then toggle Limit Ad Tracking to On.
I’m glad to see this, but even the learn more link doesn’t give you a good idea of when you are and aren’t going to be tracked. Wouldn’t it be something if Apple added an indicator in the status bar, like the location services one, that would tell you when your data is being sent to an ad server? You could then, like in the locations services preference, decide which apps could send that data, wouldn’t that be great?
Never going to happen, but interesting toggle by Apple, I just wish it was limited from the outset.